Activity for servicing assets and the related changes in fair value for the years ended December 31, 2019, 2018 and 2017 is as follows:

	2019			2018			2017
Beginning balance	$	19,693		$	21,400		$	21,091
Additions, net		6,417			7,562			6,250
Changes in fair value		(6,639	)		(9,269	)		(5,941	)
Ending balance	$	19,471		$	19,693		$	21,400

Loans serviced for others are not included in the Consolidated Statements of Financial Condition. The unpaid principal balances of these loans serviced for others were as follows as December 31, 2019 and 2018:

	2019		2018
Loan portfolios serviced for:
SBA guaranteed loans	$	1,231,959	$	1,151,915
USDA guaranteed loans		119,047		106,184
Total	$	1,351,006	$	1,258,099

Loan servicing revenue totaled $10.7 million, $10.3 million, and $9.6 million for the years ended December 31, 2019, 2018, and 2017, respectively.

Loan servicing asset revaluation, which represents the changes in fair value of servicing assets, totaled downward valuations of $6.6 million, $9.3 million, and $5.9 million for the years ended December 31, 2019, 2018, and 2017, respectively.

The fair value of servicing rights is highly sensitive to changes in underlying assumptions. Changes in prepayment speed assumptions have the most significant impact on the fair value of servicing rights.

Generally, as interest rates rise on variable rate loans, loan prepayments increase due to an increase in refinance activity, which may result in a decrease in the fair value of servicing assets. Measurement of fair value is limited to the condition existing and the assumptions used as of a particular point in time, and those assumptions may change over time. Refer to Note 18—Fair Value Measurement for further details.